This invention relates to a system and method including a waveform for addressing and achieving gray scale for an electroluminescent (EL) display. This method is suitable for high-information content EL displays, including high definition television (HDTV).
An electroluminescent (EL) display consists of a luminescent or phosphor material sandwiched between opposing arrays of electrodes. The luminescent or phosphor material is electroluminescent and emits light when the material is stimulated with an electric current from voltages applied to the opposing arrays of electrodes. The electrode arrays may be segmented for alpha-numeric characters or orthogonally arranged as opposing rows and columns to produce a dot matrix display. The voltage applied to the electrodes may be AC or DC. If the electrodes are embedded into the material as transistors or diodes, the EL display is called an active matrix EL.
The crossover of an opposing row electrode and opposing column electrode defines a cell or pixel. In a color EL display, three pixels (red, blue, green) define a picture element.
The luminance (sometimes called brightness) of each pixel is dependent upon the magnitude of the voltage applied across the particular row electrode and column electrode that defines the pixel.
The magnitude of the minimum voltage required across the pixel to cause the phosphor material to emit light is the threshold voltage.
Electroluminescent (EL) displays have been described in the prior art. The following prior art pertaining to EL displays is hereby incorporated by reference.
U.S. Pat. No. 4,636,934 (Tohda et al)
U.S. Pat. No. 5,550,557 (Kapoor et al)
U.S. Pat. No. 5,627,556 (Kwon et al)
U.S. Pat. No. 5,714,274 (Sugiura et al)
U.S. Pat. No. 5,805,124 (Kapoor et al)
U.S. Pat. No. 5,812,104 (Kapoor et al)
U.S. Pat. No. 5,838,289 (Saito et al)
U.S. Pat. No. 5,847,516 (Kishita et al)
U.S. Pat. No. 5,786,797 (Kapoor et al)
U.S. Pat. No. 4,975,691 (Lee)
U.S. Pat. No. 6,104,367 (McKnight)
This invention relates to a dot matrix EL display with opposing arrays of electrodes arranged as rows and columns.
This invention particularly relates to providing gray scale in an alternating current (AC) electroluminescent (EL) dot-matrix display consisting of luminescent or phosphor material sandwiched between opposing arrays of electrodes, the electrodes of each array being orthogonal to the electrodes in the opposing array so as to provide opposing electrode rows and electrode columns. When AC voltages are applied to the opposing electrode rows and electrode columns, the luminescent or phosphor material is stimulated with an electric current and light is emitted.
This invention has advantages over the traditional method of achieving gray scale in an electroluminescent display, which include:
This invention allows for a large number of gray scales at video refresh rate
This invention allows for the use of the xe2x80x9con-offxe2x80x9d EL cell state and easy implementation of column electrode drive circuitry. In the prior art, on off-set voltage (positive or negative) is required on the column drive circuitry. In this invention, a ground state is used to turn on a pixel to save up to 60% of the switching power.
This invention allows for use of conventional integrated circuit (IC) drivers containing no gray scale intelligence. In the prior art, EL manufacturers have used IC drivers with gray scale intelligence built into the IC driver. These IC drivers are limited to six bits or less of gray scale and have limited current capacity not suitable for large area EL displays.
This invention combines both voltage amplitude and voltage time modulation in a single waveform. The prior art uses one or the other, but not both.
This invention makes use of a symmetric scan voltage to avoid the problem with EL display panel that it often suffers from latent imaging and pseudo persistence problems which cause smearing and ghost image on the display panel. This is a result of the pixel""s voltage-time average being non-zero when averaged over several scans through the panel.
This invention relates to:
1. A system and method for driving an EL display and providing gray scale comprising a generator for generating and applying a voltage waveform to the row electrodes and column electrodes comprising:
(a) At least one Positive Ramped Modulating Pulse with a gradually sloped portion divided into n weighted divisions of xe2x80x98mxe2x80x99 sloped that are uniquely selectable in time, and may be combined in a maximum of 2n unique combinations of gray scale levels, and
(b) Zero or more Positive Non-Ramped Modulating Pulses (flat plateau) that are not sloped
These pulses, (a) and (b), when applied successively form a scan pulse, across an electrode row and an electrode column of an EL display to provide sufficient current for a sufficient time to cause the EL to emit bursts of light proportional to the current and integratable by the human eye into unique gray levels.
2. The inversion of these pulses (a) and (b) to form a Negative Ramped Modulating Pulse and a Negative Non-Ramped Modulating Pulse which negates the buildup of a voltage bias across the EL material caused by (a) and (b).
3. In the practice of this invention to provide gray scale, ramped modulation and non-ramped modulation voltages when gated by the column driver outputs will produce the luminance proportionate to the magnitude of the voltage applied across the particular row and column electrodes. Non selected columns will either have negative or positive cancellation voltage depending on the polarity of the scan. This invention also includes tri-stating the outputs of the unselected columns.
4. In this invention the voltage ramp exhibits a slope that is set to establish constant current flow through the phosphor for a given level of gray scale.